Protective headgear and helmets have been used in a wide variety of applications and across a number of industries including sports, athletics, construction, mining, military defense, and others, to prevent damage to a user's head and brain. Contact injury to a user can be prevented or reduced by helmets that prevent hard objects or sharp objects from directly contacting the user's head. Non-contact injuries, such as brain injuries caused by linear or rotational accelerations of a user's head, can also be prevented or reduced by helmets that absorb, distribute, or otherwise manage energy of an impact. This may be accomplished using multiple layers of energy management material.
Conventional helmets having multiple energy management liners are able to reduce the rotational energy transferred to the head and brain by facilitating and controlling the rotation of the energy management liners against one another. Some conventional helmets, such as, for example, those disclosed in US Published application 20120060251 to Schimpf (hereinafter “Schimpf”) employ a continuous surface interrupted by a recess in the outer liner that a projection from the inner liner extends into. Additionally, other conventional helmets, such as those disclosed in US Published application 20010032351 to Nakayama (hereinafter “Nakayama”) employ an inner liner and an outer liner that both have interlocking recesses and projections.
Some conventional helmets employ structures or objects that bridge energy liners that must break, deform, and/or deform an elastic material for the liners to rotate against each other. Such a method of energy absorption has advantages and disadvantages; while the energy is absorbed by the failure or deformation of the projections, it either happens over a short period of time, thus doing little to attenuate the rotational accelerations experienced by the user's head and brain, or the liners may tend to rotate out of one another, reducing the helmet stability.